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Transcript

MRI SYSTEM COMPONENTS
Module 1
1
SIX MAIN COMPONENTS
OF MRI SYSTEM
• Magnet
• Gradient Coils
• RF Coils
• Electronic Support System
• Computer
• Display
2
The magnet
The magnet applies a static
(homogeneous) magnetic field which
align and precess the nuclei in the body.
3
1
Gradient Coils
The gradient coils apply a
variant of magnetic field
strengths over the patient.
4
RF Coil or
Antennae
When radio
frequencies are
applied, the
atoms absorb it
and net
magnetization
changes.
5
Electronic Support System
6
2
Magnet Cross-Section
Y gradient
coil
Z gradient
coil
Transceiver
X gradient
coil
Main
coil
Patient
7
8
9
3
10
OVERVIEW of the MRI SYSTEM
MAGNET
A magnet used for MRI
must provide a large enough
opening to comfortably fit a
patient and a high degree of
magnetic field homogeneity.
The magnet’s purpose is to
align the net magnetization
of the patient’s protons and
to establish the protons’
resonant frequency.
MRI Core
11
PULSE SEQUENCE
CONTROLLER
The pulse sequence
controller is responsible for
the timing and performance
of each system component.
The pulse sequence
controller dictates when and
how much gradient power is
needed to vary the magnetic
field and spatially encode the
MR signal.
12
4
PULSE SEQUENCE CONTROLLER
The pulse sequence controller
also dictates when the RF energy
must be transmitted and for how
long.
The signal power amplifier must
be converted into an analog
continuous waveform.
The conversion is performed by
the digital- to- analog converter
(DAC).
13
DIGITAL-TO-ANALOG
CONVERTER DAC
The digital-to-analog
converter is responsible for
converting the digital
instructions from the pulse
sequence controller into a
continuous analog wave
form that is then passed
through the RF power
amplifier.
14
ARRAY PROCESSOR
The array processor is used
to reconstruct images from
raw data.
It is in the array processor
that the Fourier transform
is applied.
15
5
Prescan
• These tasks must be done before scanning can be
done:
– Tune coil
– Shim magnetic field
– Set center frequency
– Adjust transmit attenuation (RF Power Level)
– Adjust receiver attenuation (Receiver Gain)
• Failure to properly tune and match the coil may
result in noisy images with poor contrast.
16
The strength of a magnetic field is
measured in units of induction, either
Tesla or Gauss.
One Tesla equals 10,000 Gauss equals 10 kilogauss.
17
PERMANENT MAGNET
Permanent magnets are constructed from
hundreds of permanently magnetized
ceramic bricks.
18
6
The bricks are assembled such that their magnetic
fields all face in the same direction.
Once assembled, their magnetic fields add together
to make a strong enough field to perform MRI.
19
PERMANENT MAGNET
•The two magnetic field are connected to an iron
frame which acts to support the weight of the
magnet as well as focus and constrain the field.
•This configuration establishes a vertical
magnetic field, the most common permanent
magnet designs.
20
Permanent Magnet
• Costs less to operate
• Allow larger bore size
• Accommodate larger
patients
• Less chemical shift
• Smaller space requirements
•
•
•
•
Heavy
Field to about 0.3 tesla
Cannot be “turned off”
Require air-conditioning at
a constant temperature to
keep stable
• Require longer scan times
21
7
ELECTROMAGNET
Electromagnets are made from coils of wire
through which an electric current is passed.
22
23
ELECTRO or RESISTIVE MAGNET
24
8
wire
FIELD
By passing an electrical current through a coil made from
looped niobium titanium wire, a magnetic field through
the center of the coil is generated.
Both resistive and superconducting magnets share this
basic principle.
25
A basic law of magnetism states that if a charged particle
is moved a distance along a path, a magnetic field will be
generated perpendicular to the direction of the particle’s
motion.
The direction of the magnetic field, when current flows
through a coil of wire, is determined by the direction of
the current.
26
•To create strong electromagnetic fields, large amounts
of currents must be used.
•The resistance built up in the wire will produce heat
reducing the efficiency of the current and the reduction
of the magnetic field through heat loss.
27
9
RESISTIVE MAGNET
•Resistive magnets are electromagnets consisting of an air core
or iron core wrapped with a long coil of wire.
•Resistive magnets may be designed to be vertical or horizontal
magnetic fields up to about 0.3 Tesla in strength.
•Field strength is limited by the amount of power needed.
•A benefit of resistive magnets is the ability to shut the
magnetic down quickly by simply turning off the power supply.
•A drawback to resistive magnets is the energy usage and water
cooling requirements.
28
RESISTIVE MAGNET
• Can be quickly turned
off
• Low- to mid- magnetic
field
• High power
consumption
• Water cooling is
required
29
SUPERCONDUCTIVE MAGNET
30
10
•The entire system must be super cooled using liquid helium.
•By reducing the temperature down to near absolute zero,
there is virtually no resistance in the wires.
•Stronger magnetic fields can be obtained with a
superconducting magnet.
31
SUPERCONDUCTING MAGNETS
•Superconducting magnets are electromagnets
super-cooled to near absolute zero.
•The coil of wire is made of Niobium Titanium.
32
CRYOGENS
33
11
SUPERCONDUCTIVE MAGNETS
• Large service
requirements
• Cryogen maintenance
requirements
• Large magnetic fringe
field
• Magnetic field can be
turned off
• Low power
consumption
• High magnetic fields
• Stable magnetic field
with homogeneity
34
Quench
•“Unexpected loss of superconductivity in a
superconducting magnet that causes heating and very
rapid vaporization of the cryogens such as liquid
helium.
•This can cause damage to the magnet and can force
the atmosphere out of the scanner room potentially
causing anoxic conditions.”
35
Magnet Components
36
12
Magnetic Shielding
• Magnetic shielding assures that no one comes
within the 5 Gauss limit line before going
through the proper screening procedures.
• With shielding the fringe field drops off to
approximately the 5 Gauss limit line when
outside of the scan room.
37
Magnetic Shielding
• Magnetic fringe fields must be minimized for
patient safety and can be compensated for by
the use of magnetic shielding.
– Passive-shielding
– Active-shielding
– Self-shielding
38
Shielding Design
• Passive-shielding uses steel in the walls of the
scan room.
• Active-shielding uses solenoid magnets
outside the cryogen bath that restrict the
magnetic field lines to an acceptable location.
• Self-shielding uses steel in the magnet
housing.
39
13
RF Shielding
• RF Shielding assures that radio frequencies in
the outside environment do not penetrate the
MR scan room.
• Copper and stainless steel are used to create a
Faraday cage inside the scan room to assure
that no stray radio frequencies get in or out of
the scan room.
40
The Shims
•Shimming is the process by which
magnetic field inhomogeneities are
greatly reduced.
•Shimming can be accomplished in two
ways: passively and actively.
41
PASSIVE
SHIM
Passive shimming uses iron plates
arranged at specific locations on the
surface of the cylinder.
42
14
ACTIVE
SHIM
Coils of various geometry are selectively
energized in order to produce local
changes in the magnetic field where
necessary.
43
GRADIENT COILS
44
Gradient magnetic fields are used to spatially
vary the magnetic field from one point in
space to another.
45
15
46
47
GRADIENT
COILS
•The gradient coils are thick bands of conductive
material wrapped around a cylinder that fits inside the
shim cylinder.
•There are three sets of coil pairs wrapped onto the
cylinder’s surface.
•There are also three gradient power amplifiers that
drive electrical current through the gradient coils.
48
16
RF
or
IMAGING
COILS
The patient is placed in a
RF coil.
In its simplest form the RF
coil is a loop of wire which
acts as an antenna.
49
IMAGING COILS
• The closer a coil is to the area to be excited, the
less RF energy needed to create transverse
magnetization.
•The closer the receiver coil to the excited volume,
the more signal detected.
•Therefore, surface coils improve the signal-tonoise ratio(SNR).
50
COIL FUNCTION
•Two types of RF coils: transmitter coils and receiver coils.
•A single RF coil used for both transmitting the RF energy and
receiving it .
•One coil as a transmitter and a second coil as a receiver.
•The transmitter coil must be large enough and positioned and
shaped in such a way as to distribute the RF energy uniformly.
•When two different coils are used the coils must be decoupled
or electrically isolated from one another so that energy is
applied to only one coil at a time.
51
17
RECEIVER COILS
52
Volumetric Coils
•A volume coil both transmits RF and receives MR signal and
is often called a transceiver.
•Even though volume coils are responsible for uniform
excitation over a large area, because of their large size produce
images with a lower SNR than other types of coils.
•The signal quality produced by volume coils has been
significantly increased by doubling the coil sets within the
imaging coil creating quadrature excitation and detection.
•This enables the signal to be transmitted and received by two
pairs of coils.
53
Helmholtz Coil
•A Helmholtz configuration can be described as two
coils working in tandem.
• The Helmholtz pair differs from the quadrature coil
in that it is actually two linear coils.
•The purpose is to improve the signal through a
volume of tissue.
•Only useful in horizontal magnetic fields.
54
18
Solenoid Coils
• Solenoid coils are used with vertical field
magnets.
• Most are separate transmit and receive coil
configurations.
• Cannot be used with horizontal magnetic
fields.
55
Quadrature / Multi-channel
Coils
• Use two or more sets of coils – sensitive to signal
from only one polarization.
• Increased signal-to-noise, compared to linear coils.
•Increase speed of reception.
•Quad Coil can be transmit and receive coils.
•Less susceptible to artifacts when tilted.
56
Phased Array
•Several small coils in a coil holder is an
array coil.
•Can switch between using one coil by itself
or by using multiple coils together for larger
coverage.
•In this way, the signal-to-noise ratios of a
small coil can be combined to image a large
area of interest.
57
19
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